Many different types of fluid mixing apparatus have been produced in the past and, although most work relatively well, some require high energy inputs or high energy input per unit volume of the fluid being mixed. Others are often relatively inefficient and have limitations to applications. The problem is particularly acute in connection with the mixing of stratified fluid or solid/liquid suspensions or slurries.
One type of device used in the past works on the principle that a fluid vortex ring generated in the fluid will propagate through the fluid, even if it is stratified. Thus it will produce effective mixing by the transfer of mass as parcels of fluid and also create fluid motion or convection within the bulk of the fluid being mixed. That is, by impulsively moving a generator plate, having an orifice, in a controlled manner, fluid is ejected through the orifice and becomes a vortex ring.
A vortex ring is a toriodal rotating mass of fluid which has an overall oblate spheroid shape. A vortex ring efficiently transfers fluid from it's generating location to a distant location in a fluid to be mixed. Thus, mass is transferred and, when this takes place, movement of the surrounding fluid (mass convection) also takes place. Vortex rings can be highly energetic and when they impact a solid boundary cause considerable disturbance and turbulence. This is particularly useful for the agitation of sedimented slurries when an impacting vortex ring will cause dispersion of the sedimented materials. This results in low mixing times and low energy inputs, while producing homogeneity of the fluid being mixed.
Apparatus for generating vortex rings in a fluid which creates mixing of that fluid is described in the following patents issued to the present inventor: U.S. Pat. No. 5,100,242; U.S. Pat. No. 5,052,813; Canadian Patent No. 1314041; Canadian Patent Application No. 2,013,558; and European Patent No. 0 283 307B1. While the devices described in these patents work very well, a problem that sometimes occurs in the use of these devices is that a vortex ring generated on the upper surface of the generating plate is often too energetic when it reaches the free surface of the liquid. This can result in gas or air entrainment which is often undesirable. It also can result in undesirable foaming of the fluid at the surface and can cause excessive splashing. It also reduces the efficiency of the mixing because the mass of fluid within the vortex ring is not properly dissipated and distributed at the surface. The energetic breaking of the surface of the liquid results in localized turbulence whereas controlled destruction of a vortex ring, either during its forward progress or at a location just prior to the free surface, can result in highly augmented fluid circulation. This is also applicable to vortex rings energetically impacting a solid boundary, such as the base of mixing vessel, which may be useful for highly sedimented slurries etc. but may be inefficient for non-sedimentary fluid when the gradual destruction and dissipation of the material in a vortex ring is desirable.